Generally, a thyristor is a bistable power semiconductor device that can be switched from an off-state to an on-state, or vice versa. Power semiconductor devices, such as thyristors, high-power bipolar junction transistors ("HPBJT"), or power metal oxide semiconductor field effect transistors ("MOSFET"), are semiconductor devices capable of controlling or passing large amounts of current and blocking high voltages. Thyristors are generally known and conventionally have three terminals: an anode, a cathode, and a gate. A thyristor is turned on by applying a short current pulse across the gate and the cathode. Once the thyristor turns on, the gate loses its control to turn off the device. The turn off is achieved by applying a reverse voltage across the anode and the cathode. A specially designed gate turn-off thyristor ("GTO"), however, is typically turned off by a reverse gate pulse. The GTO thyristors generally start conduction by some trigger input and then behave as diodes thereafter.
A thyristor is a highly rugged device in terms of transient currents, di/dt and dv/dt capability. The forward voltage (V.sub.F) drop in conventional silicon thyristors is about 1.5 V to 2 V, and for some higher power devices, about 3 V. Therefore, the thyristor can control or pass large amounts of current and effectively block high voltages (i.e., a voltage switch). Although V.sub.F determines the on-state power loss of the device at any given current, the switching power loss becomes a dominating factor affecting the device junction temperature at high operating frequencies. Because of this, the maximum switching frequencies possible using conventional thyristors are limited in comparison with many other power devices.
Two of the most critical parameters for a thyristor are the built-in potential (which is a characteristic of any given semiconductor material's bandgap) and the specific on-resistance (i.e., the resistance of the device in the linear region when the device is turned on). The specific on-resistance for a thyristor preferably should be as small as possible so as to maximize current per unit area for a given voltage applied to the thyristor. The lower the specific on-resistance, the lower the V.sub.F drop is for a given current rating. The minimum V.sub.F for a given semiconductor material is its built-in potential (voltage).
Conventional thyristors are manufactured in silicon (Si) or gallium arsenide (GaAs), such as a silicon controlled rectifier ("SCR"). Thyristors formed in Si or GaAs, however, have certain performance limitations inherent in the Si or GaAs material itself, such as the thickness of the drift region. The largest contributory factor to specific on-resistance is the resistance of the thick low-doped drift region of the thyristor. As the rated voltage of a thyristor increases, typically the thickness of the drift region increases and the doping of the drift region decreases. Therefore, the resistance of the drift region increases dramatically. Hence, the thickness of the drift region should be minimized, and the level of doping maximized, for any given rated voltage so as to minimize the specific on-resistance for the device.
The problems with on-resistance have been recognized and several thyristor structures have been developed in an attempt to solve the on-resistance problems. These prior attempts to solve the problem included various structures of the Si or GaAs semiconductor material to try to lower the on-resistance. These prior attempts, however, were limited by the inherent characteristics of the Si or GaAs semiconductor material itself. For example, the doping level required to withstand a given voltage in a Si or GaAs thyristor is relatively low as compared to that required of a higher breakdown electric field material such as SiC. As a result, in order to form higher power thyristors in silicon or gallium arsenide, the doping in the appropriate portions of the device must be maintained at relatively low levels. This, in turn, requires that these portions be physically thicker, which makes for a generally disadvantageous specific resistance.
As a semiconductor material, silicon carbide offers a number of advantageously unique physical and electronic properties. These include its high melting point, high thermal conductivity, radiation hardness (particularly to neutron radiation), wide bandgap, high breakdown electric field, and high saturated electron drift velocity.
Recently, the common assignee of the present invention has developed various techniques for fabricating semiconductor devices from SiC, a wide bandgap material, which were previously unknown. Silicon carbide is a less mature semiconductor material than Si or GaAs because of its recent development and the performance of semiconductor devices manufactured with less mature material is generally less predictable. The common assignee of the present invention has recently developed several semiconductor devices, such as P-N Junction Diodes, Power MOSFET's, JFET's, blue light emitting diodes, thereby making SiC a commercially viable alternative to devices made from Si, GaAs, and other semiconductor materials.
Therefore, it is an object of the present invention to provide an operable thyristor formed in silicon carbide and that takes favorable advantage of the electronic characteristics of silicon carbide.